Childhood diarrhea leads to 2 million deaths every year among children less than 5 years of age mainly in underserved countries. E. coli pathogens are believed to be the second most common cause of death after Rotavirus. Six categories of diarrheagenic E. coli have been known to cause diarrhea, yet, recent studies by our group have identified a new diarrheagenic category designated localized adherent E. coli (LAEC) that is significantly associated with childhood diarrhea in Colombia, South America. The long term goal of this study is to understand the epidemiology and the molecular mechanisms of pathogenesis of this new LAEC emergent pathogen. The first specific aim proposes to evaluate the frequency and association of LAEC with childhood diarrhea by conducting two international multicenter case-control studies. We hypothesize that LAEC is associated with childhood diarrhea in children less than 5 years of age living in developing countries. The second specific aim will explain how LAEC virulence factors are essential to cause disease in a piglet model of human infection. We hypothesize that the type IV bundle-forming pilus and a novel type III secretion system directing bacterial-cell invasion are directly associated with the pathogenesis of LAEC diarrhea. The last specific aim will conduct genome DNA sequencing analysis of LAEC strains to identify the genetic elements that resulted in the emergence of LAEC as a pathogen. We hypothesize that LAEC shares in common well defined virulence genes that were acquired by horizontal gene transfer. This project if significant because it will bring fundamental knowledge on the epidemiology, genetics, and pathogenesis of LAEC and will generate molecular biology tools to facilitate future studies on LAEC diarrhea epidemiology, prevention measures and vaccine development research.